Shielded direct current amplifier



J. A. COX

SHIELDED DIRECT CURRENT AMPLIFIER Original Filed Jan. 3, 1961 Nov. 10,1964 2 Sheets-Sheet 1 "I a I I 1 X 1 VARIABLE Nov. 10, 1964 J. A. cox

ED DIRECT CURRENT AMPLIFIER SHIELD Original Filed Jan. 3, 1961 2Sheets-Sheet 2 Q02 mo wow QmQmOIU mmvroza. Q Can 2% rim div-07W UnitedStates Patent 3, 1961, Ser. No. 36,467. this application Sept. 20, W62,Ser. No.

1 Claim. or. sat-s) This application is a division of application SerialNo. 80,467, filed January 3, 1961.

The present invention relates to shielded direct current amplifierswhich are notably sensitive to pick-up of unwanted signals which causenon-linearities or instabilities in the amplifiers.

One common form of direct current amplifier includes a chopper circuitwhich converts a direct current (DC) input signal to an alternatingcurrent (A.C.) signal having an amplitude proportional to the amplitudeof the DC. input signal. The chopper circuit may include an invertercomprising two pairs of electronic switches, for example, in the form oftransistors or the like having control electrodes for rendering thedevices conductive or non-conductive depending upon the polarity andphase of control signals fed thereto. The two pairs of devices arerendered alternately conductive and are connected to an outputtransformer in a manner where the DC. input si nal is alternately fed inopposite directions through the input or primary winding of the outputtransformer. The output transformer connects with the input of an AC.amplifier circuit whose output is coupled to a demodulator synchronizedwith the switching rate of the chopper circuit. The demodulator and anassociated filter network convert the AC. amplified signal to a filteredDC. signal.

The source of control signals for the electronic switches of the choppercircuit and the demodulator is preferably a square wave generatorproviding a number of separate output signals having a 180 phaserelationship. Signals having such a phase relationship are mostadvantageous ly obtained from the output windings of a transformer. heinput and output windings of the transformer are Wound in superimposedrelation on a core of magnetic material. Unwanted signals coupled byinduction be tween the square wave generator and the chopper circuit ordemodulator are readily eliminated by enclosing the latter transformerand, if necessary, other parts of the square wave generator, in aseparate housing constituting a magnetic shield. Unfortunately, however,prior to the present invention, it was difficult to eliminate orsubstantially reduce unwanted signals capacitively coupled between thesuperimposed input and output windings of the latter transformer whichsignals created unbalanced current components in the output of thechopper circuit and the demodulator which resulted in substantialnonlinearity or instability in the DC. amplifier.

it is, accordingly, an object of the present invention to provide a DC.amplifier circuit including a chopper circuit controlled by a squarewave or AC. generator having a transformer at the output thereofprovided with a unique capacitive shielding construction for minimizingor reducing capacitive coupling of unwanted signals between the windingsof the transformer unit.

In accordance with the present invention, the output transformer of theaforesaid square wave or other AG. signal generator controlling thechopper and demodulator circuits is provided with a unique capacitiveshield betwee the superimposed input and output windings thereof. Theshield of the present invention comprises a coating of a conductivematerial, such as a low melting conductive material like zinc, which ispreferably sprayed or otherwise applied over all the exposed surfaces ofthe innermost of the windings to be shielded except for a longitudinalinsulating gap which prevents the formation of a short circuit loop. inthe case where the transformer unit is a toroidal core unit, theinsulating gap is preferably formed by applying a strip of masking tapearound the outside perimeter of the partially wound core for the full360 thereof before the spraying or other coating applying operation.After the coating operation, the masking tape is removed leaving anannular insulating gap. The coating of zinc or other conductive materialis, of course, insulated from the main body of the windings to beshielded as by the insulation surrounding the wire forming the windings.

The sensitivity of many D.C. amplifiers is such that the aforesaidinsulating gap is, in many cases, of sufiicient size to allow passagetherethrough of a significant interfering electric field which wouldadversely effect the operation of the amplifier. The present inventionovercomes this diiliculty by separately shielding the insulating gap ina way which avoids the formation of a conductive bridge across thespaced longitudinal margins of the conductive coating bordering theinsulating gap. This is most advantageously accomplished by firstapplying a strip of insulating material over the insulating gap, theinsulating material being sufficiently wide to extend beyond the marginsof the gap. A narrower strip of conductive material is positioned withinthe margins of the strip of insulating material so as to shield or coverthe insulating gap. Both the coating of conductive material and thestrip of conductive material covering tne insulating gap areelectrically connected to ground or other common reference voltage pointas by means of a single bare ended conductor soldered between the stripof conductive material and only one of the longitudinal marginalportions of the conductive coating bordering the insulating gap. Thestrip of conductive material is thus isolated from direct electricalcontact with the other longitudinal marginal portion of the conductivecoating bordering the insulating gap, to prevent the formation of aconductive loop. The winding or windings which are to be shielded fromthe inner winding or windings of the transformer unit are then woundover the shielding structure just described.

Other objects, advantages and features of the invention will becomeapparent upon making reference to the specification to follow, the claimand the drawings wherein:

PEG. 1 is a simplified box diagram of a DC. amplifier in which thepresent invention has particular utility;

FIG. 2 is a circuit diagram of a part of the DO amplifier shown in FIG.1;

FIGS. 3 and 4 are perspective views showing successive stages in theprocess of fabricating the shielded transformer forming part of the D.C.amplifier of FlGS. 1 and 2;

FIG. 5 is an enlarged fragmentary view of the partially made transformerof FIG. 4;

FIG. 6 is a fragmentary broken away view of a completed transformerconstructed in accordance with the present invention;

FIG. 7 is a transverse section through the transformer of PEG. 6, takensubstantially along the section line '7 therein; and

FIG. 8 is a plan view of a completed transformer constructed inaccordance with the present invention.

Referring now to the box diagram of FIG. 1, a typical DC. amplifierincludes a source of variable DC. signal voltage to be amplified whichis coupled to the input of a chopper circuit 4. The chopper circuitconverts the DC. signal voltage into an A.C. voltage having an amplitudecorresponding or proportional to the amplitude of the DC. signal voltageinput. As will appear, the

chopper circuit includes a series of switching devices which arerendered alternately conductive under control of a source of signalvoltage fed from a square wave generator 6. The square wave generatorhas an output transformer 8 with at least one primary or input windingha and a series of secondary or output windings 8b, 8c and 3d wound on asaturable core 3. The connections made between the output windings 8band Sc and the chopper circuit are such that the voltages are applied tothe chopper circuit from these windings 180 out of phase. The customarychopper circuit used in D.C. amplifier circuits requires that theseconnections all be ungrounded, that is floating with respect to ground.in this environment, the problem of capacitive coupling of signals fromthe input winding 3a to the chopper circuit via the output windings 3band do becomes so significant that they can very seriously adverselyatfect the operation of the D.C. amplifier system. The inventionprovides a unique shielding construction 9 diagrammatically illustratedin FIG. 1 which minimizes or eliminates this capacitive coupling.

The AC. output of the chopper circuit i is fed to a D.C. amplifier Illand then to a demodulator circuit 12 which converts the AC. voltage to apulsating direct current voltage. modulator is operated in synchronismwith the chopper circuit by means of floating connections from theoutput winding 8d of the transformer 8 of the demodulator circuit. Thepulsating D.C. signal is then filtered by a suitable filter circuit 14to provide the resulting amplified D.C.

signal.

The specific nature of the D.C. amplifier system can, of course, bevaried widely and the components thereof just described can be any oneof a number of well known types. For purposes of illustration only,exemplary cirsquare wave generator, deshown in FIG. 1 are illuscuitdetails for the chopper, modulator and filter circuits trated in FIG. 2.

The chopper circuit as illustrated includes a first pair of PN?transistors T1 and T2 and a second pair of PN? transistors, T3 and T4.The collector electrodes 16 and 18 of the transistors T1 and T3 areconnected by a conductor. 26 to the negative terminal 22 of the sourceof variable DC. signal voltage 2. The collector electrodes 21 and 23 ofthe transistors through a conductor 24 to the positive terminal 26 ofthe variable D.C. signal source. The emitter electrodes 28 and 30 oftransistors Tl and T2 are connected together by a conductor 31 and theemitter electrodes and 34 of the transistors T3 and T2 are connected to-U The latter conductor 3% is connected by a conductor 36 to one end ofthe input winding 38a of an output transformer 35%. The conductor 31connecting the emitter electrodes 2% and 36* of the transistors T1 andT4 are connected through a conductor to the other end of the inputwinding 3311.

As will appear, when the first pair of transistors T1 and T2 arerendered conductive, the path for current flow through the choppercircuit from the negative terminal 22 of the variable D.C. signal source2 can be traced through the conductor 2h, collector and emitterelectrodes l6 and 28 of the transistor T1, conductors 31 and 40, theinput winding 38a in a direction from the bottom to the top terminalsthereof, conductor 35, the emitter and collector electrodes 34 and 21 oftransistor T2, and conductor 24 leading to the positive terminal 26 ofthe variable D.C. signal source. When the second pair of transistors T3and T4 are conductive, current flow can gether by a conductor 35.

be traced in a path extending from the negative terminal 22 through thecollector and emitter electrodes 18 and,

32, of transistor T3, conductor 36 leading to the upper end of the inputwinding 38a, conductor 4t emitter and collector electrodes fill and 23of transistor T4.- and the conductor 24 leading to the positive terminalis.

As previously indicated, the means for opening and in the manner to bedescribed, the de- T2 and T4 are connected i 43 of transistor T2. Thewindings closing the electronic switches formed by the transistordevices Tl-TZ and Tfi-Td includes control signals from the square wavegenerator 6. The transistors are rendered conductive and non-conductiveby the feeding of suitably phased voltage to the base electrodes i1 and43 of transistors T1 and T2 and base electrodes 43-5 and 4'7 oftransistors T3 and T4. The upper terminal of output winding 8b of thesquare wave generator transformer 25 is coupled by a conductor SQ to aresistor 52 connected to the base electrode 41 of transistor Til. Theupper terminal of the output winding 30 is coupled by conductor 54 to aresistor 55 connected to the base electrode 8b and 80 have center tappedpoints respectively connected by conductors 57 and 59 to the commonlyconnected collector electrodes of transistor pairs Tl-T3 and T t-T2. itis thus apparent that the phase of the induced voltage at the upperterniinals of output windings 8b and tic is identical and that the"transistors Til and T2 are simultaneously rendered conductive andnon-conductive during successive half cycles of the square wave outputof the transformer 8.

The bottom terminal of output winding 99b is coupled i by a conductor $8to a resistor 66 connected to the base electrode of transistor T3. Thebottom terminal of the output winding be is coupled by a conductor 62 toa resistor tid connected to the base lectrode 4-7 of transistor T4. itis likewise apparent that transistors T3 A and T4 will be renderedsimultaneously conductive and non-conductive alternately with thefirst-mentioned pair of transistors T1 and The square wave generator 6illustrated in the drawings includes a pair of NPN transistors T5 andT6. These transistors have collector electrodes 65 and 6? connectedthrough conductors 69 and il to opposite ends of the transformer inputWinding 8a. The input winding has a center tap point 73 connected to thepositive terminal of a source of direct current voltage 74, the negativeterminal of which is grounded. The transistors T5 and T6 have emitterelectrodes in and 78 respectively connected to a ground conductor $0.The ground conductor extends to the upper terminal of a feedback orcontrol winding 8e wound on the core 8 of the transformer unit 8. Thebottom terminal of the winding Se is connected through a resistor $3 ofthe base electrode of the transistor T5. The ground conductor ht alsoextends to the bottom terminal of a second feedback or control winding8' Whose upper terminal is connected through a resistor 8'"? to the baseelectrode 3? of the transistor To. A capacitor-resistor network 87 isconnected between the collector electrode 65 of transistor T5 and thebase elec trode 89 of the transistor rs. A similar capacitor-resistornetwork 8% is connected between the collector electrode er of thetransistor T6 and the base electrode 85 of the transistor T5. Thesefeedback networks aid in reducing the change-over time when theconductive condition of the transistors T5 and T6 reverse. When one ofthe transistors T5 intially becomes conductive, the resulting iiow ofcurrent through the input winding 800 generates a feedback voltage inthe feedback winding 8e which maintains the conduction of the transistorT5. Conversely, the voltage induced in the other feedback winding 8 atthat instant is in a direction which keeps the transistor T6non-conductive. The core S of the transformer unit $5 is made of arectangular hysteresis material and when this material saturates, thesense of the voltages then induced in the feedback windings 8e and 3freverses to trigger the then non-conductive transistor into a conductivestate and the conductive transistor into a non conductive state. It canbe shown that the output voltage induced in the output windings 8b, ticand 8d is substantially a square wave as illustrated in FIG. 2.

As previously indicated, the chopper circuit 4 provides a flow ofalternating current in the input winding 33a of the output transformer38 whose amplitude is proportional to the amplitude of the input D.C.signal voltage fed from the source 2. Transformer 38 has an outputWinding 38b feeding the input of an AC. amplifier which may be aconventional type amplifier. The amplifier 10 has an output transformerwith an input winding 90a and an output winding 9015 which feeds theinput of the demodulator circuit 12.

The demodulator circuit includes a pair of rectifier bridge networks 92and 92. The bridge network 92 includes a first pair of rectifiers 92aand 92b connected in series in the same sense between a pair of oppositebridge terminals 94-96. It also has a second pair of rectifiers 92 and92d which are connected in series in the same manner between theterminals 94 and 96.

The other bridge network 92' comprises a pair of rectifiers 92a and 92bconnected between terminals 94' and 96' but arranged in the oppositesense to the correspond ing rectifiers 92a and 92b in the other bridgenetwork 92 so that the path for current flow is between terminals 94 and96' instead of between 96' and 94. The second bridge network includes asecond pair of rectifiers 92c and 92d which are connected in series inthe same sense as rectifiers 92a and 9212' between the terminals 94 and96'.

The bridge network terminals 94 and 94 are connected through respectiveresistors 96 and 9a to a common conductor 98 extending to the bottomterminal of the output winding 8d of the square wave generatortransformer 8. The bridge network terminals 96 and 96' are connectedthrough respective resistors Tilt? and 1th) to a common conductor 192extending to the upper terminal of the transformer output winding 8d.

The upper terminal of the amplifier output transformer winding 9% isconnected by a conductor 1% to the juncture between rectifiers 92a and92b of bridge network 92 and the bottom terminal of the latter windingis connected by a conductor 106 to the juncture between the rectifiers92c and 92d of the bridge network 92.

The juncture between the other pairs of diodes 92c-92d and 926-9242" ofthe two bridge networks are connected to a common conductor 1il7extending to one of the inputs of the filter network 14. The amplifieroutput transformer winding 90b has a center tap point which is connectedby a conductor 199 to the other input of the filter network 14. Theinput conductor 109 extends to a series circuit of a resistor 111, afilter choke 113 and a filter choke 115 leading to an output terminal117 of the filter network. The other input conductor 107 to the filternetwork extend to the other output terminal 119 of the filter network.Filter capacitors 121 and 123 are connected between the opposite sidesof the filter choke 115' and the input conductor 107.

It is apparent that the frequency of the signal in the amplifier outputtransformer winding 90b and the control signal fed to the demodulatorcircuit from the square wave generator transformer winding 80! isidentical, the amplitude of the former signal varying with the amplitudeof the variable input DC. signal and the output of the latter signalbeing constant. The polarity of the alternating current signals fed fromthese two sources to the demodulator circuit also change at the sameinstant of time. It can be shown that the demodulator circuit justdescribed is so designed that the alternating current output from thetransformer 90 is converted to a constant DC. signal at the output ofthe filter network 14 having an amplitude proportional to that of thevariable D.C. input signal delivered by the signal source 2.

It can be appreciated that the useful signals coupled between theprimary winding 8a of the square wave generator transformer 8 and theoutput windings 8b, 8c and 8d are inductively rather than capacitivelycoupled. Unwanted signals inductively coupled to the chopper circuit anddemodulator circuit can be avoided by enclosing the square wavegenerator in a separate housing made of magnetic shielding material. Anysignals which are capacitively coupled between the input and theoutputwindings of the transformer would also adversely atfect the DC.

amplifier by creating unbalanced current components in the system whichwould result in instabilities or nonlinearities in the characteristicsof the DC. amplifier system.

One aspect of the present invention deals with the particular means forproviding a shield between the input winding 8a and the output windings3b, 8c and 8d of the square wave generator transformer. Some of theproblems in the design of this shielding are related to the extremelysensitive nature of the DC. amplifier system which would not normally bepresent in many other circuit environments.

Refer now to FIGS. 3 through 8 which show the construction of the squarewave generator transformer 8. The transformer has a toroidal core 8'made of a rectangular hysteresis core material. The input winding ha maycomprise a wire 8a having a suitable covering or coating of insulationdo" as in the case of conventional insulated wire used in thefabrication of transformer windings. The insulated wire 8a is woundaround the core 8' in a conventional way and may constitute one or morelayers of wire turns extending part way around or completely around thetoroidal core. The feedback windings 8e and 8 may, if desired, occupy aposition around or beneath the turns constituting the input winding 8aor they may be wound around different segments of the toroidal core 3not occupied by the input winding 8a, where the latter does not extend afull 360. These details, of course, have nothing whatever to do with thepresent invention.

The shielding 9 between the input and output windings of the transformerincludes a coating 124 of conductive material applied over the innermostof these windings, the input winding 8a in the exemplary form of theinvention being described (and the other windings 8e and 8; where theyconstitute inner windings of the core along with winding 8a). Theconductive coating, most advantageously, is zinc sprayed in molten formover the entire exposed surface area of the core unit before the outputWindings 8b, 8c and 8d are applied, except for a peripheral annularinsulation gap 125 extending all the way around the core unit. Theinsulation gap 125 prevents the formation of a short circuit loop whichwould adversely etfect the operation of the transformer. The insulationgap 126 is most advantageously formed in the manner illustrated in FIG.3. Before the molten zinc coating is sprayed on the core unit, a strip128 of masking tape is secured around the outside of the partially woundcore unit. Also, prior to the application of the molten zinc, a windingof Mylar or similar insulation is wound around the partially wound coreunit to protect the insulating coating 8a", etc. of the subjacentwinding or windings from the hot zinc which could destroy the coating.The winding 125 can be omitted Where the insulation 3a" is not adverselyaffected by the application of the coating 124. Then the entire exposedsurface of the core unit is sprayed with Zinc and the masking tape 123is then stripped from the core to leave the continuous insulating gap126. In one embodiment of the invention, the insulating gap had a widthof A of an inch. However, the exact width of the insulating gap isunimportant. Zinc is the preferable material for the conductive coating124 since it has high conductivity and a low melting tem perature whichwill not harm or destroy the masking tape 123 or other insulationmaterials beneath the coating.

Despite the fact that the insulating gap 126 occupies only a smallfraction of the area covered by the conductive coating 124, it has beenfound that for DC. amplifier applications the insulating gap 125described above provides a sufiicient space that capacitive coupling tothe output windings 8b, 8c and 3d is significant, particularly insituations requiring severe operating requirements for the DC.amplifier. To prevent such undesired capacitive coupling, the insulatinggap 126 is covered by conductive material in a manner which does notbridge the longitudinal margin portions of the conductive coating 124bordering the insulating gap. This is most effectively accompiished byfirst applying around the entire core a strip 13% of insulation materialof substantially greater width than the insulating gap 126 so that thelongitudinal margins thereof extend well beyond the gap, as shown mostclearly is 35163. 6 and 7. The strip of insulating material may be madeof Mylar insulation having an adhesive coating on the inner side foradhering the same to the conductive coating 1%.

A strip of conductive material we of tin foil or the like is adhesivelyor otherwise applied over the strip of insulating material 130 for thefull 360 of the toroidal core unit. The conductive strip 136 is somewhatwider than the insulating gap 126 so as to extend beyond thelongitudinal magins thereof, but is narrower than the strip ofinsulating material 136 so that it is located completely within thelongitudinal margins thereof.

The conductive strip 136 is electrically connected to the conductivecoating 124 by means preferably including the bared wire end portion ofan insulated conductor 142. The bared wire end portion 14-5; extendscircum ferentially around the outer portion of the core unit as shown inFIG. 7 and is soldered or otherwise electrically and physically anchoredbetween the conductive strip 130 and the conductive coating 124. Thebared wire end portion 140 is thus secured to only one of thelongitudinal marginal portions of the conductive coating bordering theinsulating gap 1%, so that the bared wire end portion and the conductivestrip are isolated from direct electric from the other longitudinalmarginal portion of the conductive coating 1.24 bordering the insulatinggap ran, to avoid providing a short circuit loop.

A layer 144 of insulation in the form of a strip of Mylar materialspirally wound around the core unit may then be applied around the coreunit to insulate the conductive strip 135 and more importantly, toprotect the windings to be tightly applied around the exposed portionsof the Zinc coating 12d from damage by their contact with the roughsurface of the zinc coating. This insulating layer 114 could be omittedwhere the insulation of the windings to be applied over the shieldingconstruction just described is not damaged by the zinc coating and isotherwise suitable as insulation.

Next, the output windings 3b, 3c and 8d are wound around the shieldingconstruction just described in different angular positions around thecore as shown in P16. 8. Individual Mylar strips 146, 148 and 15d ofinsulation are then Wound around the individual windings 8b, 8c and 8d.The various leads extending to the windings of the transformer unit areshown loosely extending from the transformer. However, these windingscan be gathered together at any suitable point or in a number ofdifferent points in a manner well known in the art. It should be furtherunderstood that additional winding layers or shielding layers may beapplied around or between the windings illustrated in the drawingswithout deviating from the basic aspects of the invention.

The shielding construction above described can be quickly and easilyapplied so that the transformers can be mass produced. The spraying ofthe zinc coating 124 is of particular value in this regard, although thebroader aspects of the invention envision the application of the coating12% by other means.

Various additional modifications may be made in the transformerdescribed above without deviating from the broader aspects of theinvention.

What I claim as new and desire to protect by Letters Patent oi": theUnited States is:

In a direct current amplifier system comprising: a pair of directcurrent signal input terminals, a chopper circuit for converting thedirect current input signals to pulsating current, the chopper circuitcomprising a first and a second pair of current conducting devices eachhaving a control terminal for rendering the associated device conductiveor non-conductive depending upon the polarity of a control voltagesignal fed thereto, and a pair of output terminals to be energized bythe direct current input signal, an output transformer having input andoutput windings, means for connecting one of said pairs of currentconducting devices between said signal input terminals and the inputwinding of said output transformer for providing a low resistance pathfor current flow through said transformer input winding in one directionwhen the associated devices are rendered conductive, means for couplingthe other pair of current conducting devices between said signal inputterminals and said input winding of said output transformer forproviding a low resistance path for current flow through saidtransformer input winding in the other direction when the associateddevices are rendered conductive, and a control voltage signal sourceincluding a signal output transformer having input winding means andoutput winding means wound in superimposed relation on a core ofmagnetic material, a control voltage signal being induced in the latteroutput winding means from the latter input winding means, and ungroundedmeans connecting said output winding means to the control terminals ofsaid first and second pair of conductive devices to render the samealternately conductive, the improvement comprising electric fieldshielding means between said input and output winding means, saidshielding means comprising an electrically conductive coating insulatedfrom said winding means and surrounding the inner of said winding meansexcept for an insulating gap extending the length of said coating, whichprevents the formation of a short circuit loop around the core, and astrip of conductive material located between and insulated from saidwindings and positioned to cover said insulating gap electrically toshield the same against passage of any significant electric fieldtherethrough from the inner winding means, said strip of conductivematerial being electrically connected to said coating but isolated fromdirect electrical contact with one of the longitudinal marginal portionsof said coating bordering said insulating gap to prevent the formationof a short circuit loop around the core.

No references cited.

